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Bolt Strength Calculator

Calculate the design shear and bearing strength of structural bolts (Black & HSFG) based on grade, diameter, and plate thickness.

Project Specifications
Bolt Specification
M16, M20, M24...
Connection Details
Thinner plate in connection
strength breakdown
Shear Strength ($V_{nsb}$): 0 kN
Bearing Strength ($V_{npb}$): 0 kN
Critical Mode: -
Calculated Output
Bolt Capacity ($V_{db}$)
0 kN
0 kN
Capcity (kN)
0 mm
Hole Size
Comprehensive Guide

Structural Bolt Design and Connections

Master the math of steel joints. Learn why we check both 'Shear' and 'Bearing' and why Grade 8.8 bolts are the industry standard for high-strength connections.

The Nut and Bolt of Steel Design

Steel buildings aren't welded in a single piece; they are assembled like giant Lego sets using bolts. A bolt in a structural joint fails in one of two ways:
1. Shear: The bolt itself is sliced through by the offset force of the plates.
2. Bearing: The bolt "crushes" the side of the hole in the plate, or the plate itself "crushes" the bolt.
The **Bolt Value** is the smaller of these two strengths. It represents the maximum load a single bolt can safely transfer.

IS 800 (LSD) Formulas

$$V_{dsb} = \frac{f_{ub}}{\sqrt{3} \times 1.25} \times n_n A_{nb}$$ $$V_{dpb} = \frac{2.5 k_b d t f_u}{1.25}$$

Common Bolt Grades

  • Grade 4.6 (Mild Steel): Standard "Black Bolts." They are cheap and reliable for low-load secondary members. The "4" means ultimate strength is $400$ MPa, and ".6" means the yield point is $60\%$ of that.
  • Grade 8.8 (High Strength): The workhorse of modern steel construction. These bolts are hardened and can carry far higher loads.
  • HSFG Bolts: High Strength Friction Grip bolts. These aren't designed to "bear" against the hole; they are tightened so hard that the Friction between plates holds the joint together.

Clearance and Holes

You cannot fit a $20$mm bolt into a $20$mm hole. Standard construction requires a "Clearance" (usually $2$mm for bolts up to $24$mm). This allows for slight misalignment during assembly on-site. The **Shear Area** ($A_{nb}$) used in calculations is actually the area at the **Threads**, which is roughly $78\%$ of the full diameter area.

Frequently Asked Questions (FAQ)

What is a 'Snug-Tight' bolt?

"Snug-tight" is the tightness attained by a few impacts of an impact wrench or the full effort of a man using an ordinary spud wrench. It ensures the plates are in solid contact. For HSFG bolts, further tightening (Torquing) is required to develop the necessary friction clamping force.